Fertilizers are added to the soil of crops or in some cases they can be applied directly to crop foliage to supply elements needed for plant nutrition. Seventeen elements are known to be essential to the health and growth of plants. Typically, nitrogen, phosphorus, and potassium are provided in the greatest quantity. With increasing knowledge of the role of each of the nutrients essential to plants, there is a better understanding of the importance of providing a given nutrient at the appropriate stage of phenology. To accomplish this, rapid changes in fertilizer formulations and methods of application have been necessary.
Another factor changing fertilization formulations and methods is due to pressure from federal, state and local regulatory agencies and citizen groups to reduce the total amount of fertilizer in general, and of specific nutrients in particular, being applied to the soil. Additionally, the loss of registration of existing synthetic plant growth regulators and organic pesticides and the prohibitively high costs involved in the successful registration of new ones, also plays a role in the changing arena of crop fertilization.
The principal source of phosphorus for the fertilizer industry is derived from the ores of phosphorus-containing minerals found in the Earth's crust, termed phosphate rock. Elemental phosphorus does not exist in nature; plants utilize phosphorus as the dihydrogen phosphate ion (H2PO4−). While untreated phosphate rock has been used for fertilizer, it is most commonly acidulated with dilute solutions of strong mineral acids to form phosphoric acid, which is more readily absorbed by crops.
Until recently, phosphate and polyphosphate compounds were considered the only forms in which phosphorus could be supplied to plants to meet the plant's nutritional need for phosphorus. Indeed, the only phosphite compound cited for use as a fertilizer in the Merck Index (M. Windhols, ed., 1983, 10th edition, p. 1678) is calcium phosphite (CaHPO3). No phosphite fertilizer formulations are listed in The Farm Chemical Handbook (Meister Publishing Co., 1993, Willoughby, Ohio 834 p.) or Western Fertilizer Handbook (The Interstate, Danville, Ill. 288 p.) Historically, calcium phosphite was formed as a putative contaminant in the synthesis of calcium superphosphate fertilizers [McIntyre et al., Agron. J. 42:543–549 (1950)] and in one case, was demonstrated to cause injury to corn [Lucas et al., Agron. J. 71:1063–1065 (1979)]. Consequently, phosphite was relegated for use only as a fungicide (Alliete®; U.S. Pat. No. 4,075,324) and as a food preservative.
More recently, it has been shown that plants can obtain phosphorus from phosphite [Lovatt, C. J., Mar. 22, 1990, “Foliar phosphorus fertilization of citrus by foliar application of phosphite” In: Citrus Research Advisory Committee (eds) Summary of Citrus Research, University of California, Riverside, Calif. pp 25–26; Anon., May, 1990, “Foliar applications do double duty” In: L. Robison (ed) Citrograph Vol. 75, No. 7, p 161; Lovat, C. J., 1990, “A definitive test to determine whether phosphite fertilization can replace phosphate fertilization to supply P in the metabolism of ‘Hass’ on ‘Duke 7’:—A preliminary report” California Avocado Society Yearbook 74:61–64; Lovatt, C. J., 1992]. Formulations based on phosphorous acid and hypophosphorous acid, as phosphite is, generally undergo oxidation to phosphate and thus lose the benefits that could be derived from the use of phosphite fertilization applications.
The phosphate and polyphosphate fertilizers currently used have a number of properties that compromise their desirability as fertilizers. Generally, they tend to form precipitates during storage and shipping. This limits the ability to formulate concentrated solutions of fertilizers. Additionally, formulations must generally be maintained at a narrow pH range to prevent precipitation, resulting in fertilizers that are limited to particular uses.
Another drawback of phosphate fertilizers is that they are not readily taken up by the foliage of many plants and must instead be delivered to the soil for uptake by plant roots. The mobility of phosphate fertilizers in the soil is limited leading to rapid localized depletion of phosphorus in the rhizosphere and phosphorus deficiency of the plant. Frequent reapplication of phosphate fertilizers is undesirable because it leads to leaching of phosphate into the groundwater resulting in eutrophication of lakes, ponds and streams.
Phosphate and polyphosphate fertilizers have also been shown to inhibit the beneficial symbiosis between the roots of the plants and mycorrhizal fungi. They tend to support the growth of algae and promote bacterial and fungal growth in the rhizosphere, including the growth of pathogenic fungi and other soil-borne pests.
Even though phosphorus, once in the plant, is very phloem mobile (i.e. readily moving from old leaves to young tissues), phosphate is poorly absorbed through the leaves of most plant species. This is unfortunate because successful foliar phosphorus feeding would result in the application of less phosphate fertilizers to the soil and reduce phosphorus pollution of the ground water.
Accordingly, there is a need for a phosphorus fertilizer that can be utilized in irrigation systems and applied to foliage without the formation of precipitates that reduce nutrient availability and uptake by the plant and plug emitters and sprayers. There is also a need for new methods of fertilizer application that allow nutrients in a readily available form to be supplied at the exact time the plant needs them. This need includes the facility of a foliar product to be sold in a single formulation for use as a concentrated material for airplane or helicopter application or as a dilute solution for ground spray application and yet able to be maintained at a suitable pH range optimal for leaf uptake despite the need to be diluted prior to application.
Additionally, there is a demand for phosphorus fertilizers that have the facility to be used as liquids or solids (granule or powder). There is also a demand for fertilizers that do more than just supply nutrients. It is desired that the fertilizers also have demonstrated plant growth regulator activity, increase the plants' resistance to pests, promote plant health in general and root health in particular, increase the production of allelopathic compounds, increase pre- and post-harvest quality, improve stress tolerance, enhance beneficial symbioses, and improve yield over existing traditional soil or foliar fertilizers.